It has been an important task in the field of paint to reduce use amount of organic solvent, for environmental preservation and resource-saving. As a means to accomplish the task of reducing organic solvent content in paint, recently development of water-borne paint and high-solid paint is promoted.
As water-borne paint in the field of automobile coating, for example, a thermosetting water-borne coating composition composed mainly of polyvalent carboxylic acid type resin, amino resin, linear low molecular weight polyesterdiol and benzoin (e.g., see JP Hei 4 (1992)-93374A) is known. However, it has a low solid content, insufficient surface smoothness and unsatisfactory coating film performance such as water resistance.
Of coating compositions for automobiles, particularly top-coating compositions are required to form coating film of excellent appearance and high quality, and in the occasions of metallic coating, it is required to provide highly appealing coating film finish such as metallic effect, by favorably orienting flaky effect pigment such as aluminum in metallic base coating composition.
As a water-borne metallic base coating composition used for such metallic coating, for example, a base coating composition which allows the maximum expression of flip-flop effect of metallic pigment and forms coating film excelling in metallic effect is proposed, which is characterized by containing fine particles of crosslinked polymer in its water-borne base paint (e.g., see JP Hei 3 (1991)-14869B). However, the composition has a defect that it is apt to induce coating film faults such as sagging, unevenness or the like under variation in coating conditions, in particular, that in humidity.
There has been also proposed a coating film-forming method by 2-coat-1-bake system, which used as the paint for water-borne metallic base coating an aqueous dispersion formed by copolymerizing an aqueous dispersion containing fine polymer particles with a specific long chain monomer such as, for example, stearyl acrylate, stearyl methacrylate and the like (e.g., see JP 2001-104878A). The method, however, is subject to such problems as insufficient coating workability and finished appearance such as surface smoothness, metallic effect and the like.